1. Field of the Invention
The present invention relates to a method of bonding metallurgical incompatible metals by electron beam welding to form an article. More specifically, the invention relates to the fabrication of a rotor for a rotating head device from aluminum and stainless steel foil.
2. Prior Art
The following U.S. Patents are representative of the prior art: U.S. Pat. Nos. 3,840,894; 3,566,008 and 3,458,683.
Also an article entitled "Air Bearing Construction" in the IBM Technical Disclosure Bulletin, Volume 16, No. 12, May 1974, page 3847 is representative of the prior art.
Rotating head magnetic devices are well known in the prior art. In the so-called helically scanned devices, a length of magnetic media is helically wrapped around a mandrel which is split and is separated to accommodate a rotating wheel, sometimes called a rotor, which carries a magnetic transducer. Generally, the mandrel comprises two separate cylindrical halves which abut the rotating wheel. The rotating wheel and the cylindrical halves have substantially identical radius, and the rotating wheel carries the magnetic head. In many applications, the magnetic media makes contact with both the mandrel surface and the rotating wheel carrying the magnetic transducer.
The ideal condition is for the magnetic media to fly relative to the rotating wheel. However, due to imperfections in the media surface, this ideal condition is not reached. That is, at some point in time the magnetic media will be in contact with the rotor. With this realization, in order to extend the wear life of the rotor it is necessary that the surface or skin of the rotor which comes in contact (i.e., the surface which interfaces) with the magnetic media be manufactured from a relatively hard material or metal. For example, stainless steel is an acceptable metal.
Due to the fact that the rotor has to be accelerated at relatively high speed, for example, 5,000 inches per second, and then stopped within a relatively short time, the rotor support to which the stainless steel foil or skin is affixed has to be manufactured from a relatively light weight material, for example, aluminum.
It is well known in the prior art that aluminum and stainless steel are dissimilar metals; i.e., the metals are metallurgically incompatible. Being dissimilar or incompatible, it is extremely difficult if not impossible to bond these metals together to form a conventional rotor which will withstand wide temperature variation, for example, within the range of from -40.degree. F to 350.degree. F.
One method used in the prior art to bond dissimilar metals (for example, a rotor having an aluminum body with a stainless steel skin or foil attached thereto) is to epoxy said metals to form a unified structure. Generally, a glue line (i.e., the interface between the two dissimilar metals) is established by placing a layer of epoxy or adhesive on one surface of the stainless steel foil and placing another layer of adhesive on the outside surface of the rotor body. The epoxy surfaces of the stainless steel foil and rotor body are then brought into contact and a clamp is used for applying pressure. The structure is then baked to a temperature of approximately 350.degree. F. The structure is then cooled to room temperature and the clamp is removed. This completes the process and the article (rotor) is now ready for use in a rotating head device. A more detailed description of the process is disclosed in the above referenced IBM Technical Disclosure Bulletin.
The article and method as described above are plagued with several problems which render the article unsuitable for its intended purpose or use.
Probably the most devastating problem is that the glue line (i.e., the epoxy interface between the stainless steel foil and the aluminum rotor body) rupture, during cooling, thereby separating the stainless steel foil from the aluminum rotor. The defect stems from the fact that stainless steel and aluminum have different coefficients of expansion. Due to this difference, as the rotor structure cools, the stainless steel and the aluminum shrink at different rates. This tends to set up shear forces within the glue line which in time separates the stainless steel from the aluminum thereby rendering the structure useless.
Another problem which plagues the prior art is that of concentricity. As was mentioned previously, in bonding the stainless steel foil to the aluminum rotor body, a layer of epoxy is used as the bonding substance. Once the structure (i.e., the rotor) is cured and cooled, it is machined to have a specified surface contour. After machining, the surface contour must be concentric with a reference point. If this concentricity is not met, then the rotor is rejected.
In order to maintain acceptable bonding strength between the stainless steel foil and the aluminum rotor body, a relatively thick layer of epoxy is deposited to form the glue line. However, the thicker the glue line the more difficult it is to maintain uniform thickness. Stated another way, the thicker the glue line the stronger the bond, but the less likely it is that the rotor's concentricity will be maintained prior to machining. The obvious solution to the problem is to deposit a relatively thin glue line. But with a thin glue line, the more likely it is for the stainless steel foil to separate from the aluminum rotor body. Faced with the above competing considerations (i.e., thick versus thin glue line), the prior art method of fabricating rotors resulted in extensive trial and error approach.
Still another problem which results from the prior art method is that of low rotor yield (i.e., the ratio of acceptable rotors to rejected ones). The low yield stems from the fact that after the rotor is cooled, it has to be machined to achieve a desired surface contour and concentricity. In machining the rotor if the glue line is not evenly distributed, then in order to maintain the desired concentricity, the stainless steel foil or skin will be unevenly machined. For example, in some cases the stainless steel foil was completely machined through (i.e., chewed away) thereby exposing the epoxy surface.
Finally, the cost of manufacturing a rotor using the prior art method is rather expensive. The expense stems from the fact that a relatively large amount of time is spent in applying the adhesive to the material (i.e., the aluminum rotor body and the stainless steel foil), a large amount of time is spent in baking the material and a large amount of time is spent in cooling the material. With this increased cost, the prior art article and method is not suitable for manufacturing a rotor for a low cost magnetic tape transport.
Although the prior art has been discussed in the preceding paragraph in view of a specific article, it should be noted that the forming of a joint between dissimilar metals or the bonding of dissimilar metals is extremely difficult. In fact, the applicants have no knowledge of prior art wherein dissimilar metals are bonded together in a non-laminate structure.
U.S. Pat. No. 3,566,008 is an example of prior art method of forming a joint between dissimilar metals. In this patent, aluminum is bonded to plated copper. In achieving the bond chamfered holes are punched within the cylindrical copperplated structure. The aluminum structure is then fabricated with tubular portions which are inserted in the chamfered holes. Heat and pressure is then applied to the copper member until the aluminum melts and flows out of the holes thus forming a joint.
Another example of the prior art joining of dissimilar metals to form a laminate structure is disclosed in U.S. Pat. No. 3,458,683. In this patent a metallic foil is sandwiched between two pieces of metallic material. An electron beam is then directed through one of the two retaining metals to thereby form a continuous weldment between steel foil and metal.